My SmartThings has worked fine for over a year. I have two GE - In-Wall Smart Dimmer Switches that now occasionally go off-line. Now neither of them work manually or automatically. I have re-booted the hub and that did not help. I do see the blue light but when I try to manually turn on the light the blue goes away.
Finally, this may have started when the hub discovered a number of items automatically and added them to the system. (mostly Sono’s)

I have two GE - In-Wall Smart Dimmer Switches that now occasionally go off-line. Now neither of them work manually or automatically. I have re-booted the hub and that did not help. I do see the blue light but when I try to manually turn on the light the blue goes away.

When you said didn’t work manually. Do you mean by physically turning the switch on/off or with the SmartThings app? If physically switching and it didn’t work then check your bulbs or switch. If you meant with the ST app then it’s possible the switch lost pairing to the hub or the “health check” in the ST app is causing the issue.
You can try turning off the health check or excluding the switch then use replace switch in the ST app again.

Log into IDE and under my hubs about half way down you will see utilities and view utilities. Click view utilities then repair z-wave network.

It’ll take a few minutes to complete, at least in my case it did and you can view the progress. I believe it says it is finished eventually. In my case it doesn’t look like it is doing anything but it is.